Switched mode power supplies as main part of telecom and commercial systems often dictate their size and electrical performance as well as reliability and costs. As requirements for the key characteristics power density and efficiency of power converters increase, the demands of these evaluation characteristics increase for inductive components particularly. One approach of increasing the power density and the efficiency is to integrate the inductive components. Transformers and inductors can be integrated into a single magnetic structure, which reduces costs, increases power density and power efficiency.
A circuit where integrated magnetics is strongly recommended is the current-doubler rectifier (FIG. 1), which can be used with different double-ended primary topologies such as forward, two transistors-forward, push-pull, half bridge or full bridge converters. The current-doubler rectifier circuit, habitually applied for low voltage and high current outputs, uses one simple two-winding transformer and two output inductors. The current-doubler rectifier then exhibits lower conduction losses than the conventional centre tapped rectifier. This configuration results, additionally to the number of discrete magnetic components which yield higher size and costs, in three high current windings and several high interconnection losses which negatively impact the efficiency.
A further circuit where integrated magnetics are strongly recommended is the LLC resonant converter (FIG. 15b), which is capable of yielding high efficiency while operating at high switching frequency. The LLC resonant converter uses three magnetic components: a series resonant inductor, a parallel resonant inductor and depending on the chosen rectifier circuit a two- or three-winding transformer. This converter results, additionally to the number of discrete magnetic components which yield higher size and costs, in at least three windings and several interconnections which negatively impact the efficiency. The parallel resonant inductor and the transformer are generally integrated into one component. An air gap is ground in the non ideal transformer in order to adjust the magnetizing inductance which replaces the parallel resonant inductance.
In U.S. Pat. No. 6,784,644 (Xu et al.), an integrated magnetic structure was introduced, where the transformer secondary winding and secondary inductor windings were integrated, resulting in the removing of the secondary transformer winding with the functionality of the rectifier being guaranteed. Due to introduction of air gap, the secondary windings not just transform but also store energy. The cores-together with the windings-integration cause the cost to be reduced and power density to be increased. The reduction of the number of secondary windings and high current interconnections result in lower winding losses. The tight coupling of primary and secondary windings yields minimized leakage inductance.
The integrated magnetic structures shown in U.S. Pat. No. 6,549,436 (Sun), U.S. Pat. No. 6,163,466 (Davila, Jr. et al.), and U.S. Pat. No. 7,034,647 (Yan et al.) comprise four windings: a primary winding, two secondary windings and an additional filter winding which is introduced to further increase the effective inductance and reduce the current ripple in the output of the current-doubler rectifier circuit.
In EP 1 895 549 (DET International Holding Limited), an inductive element with at least two core parts and at least one winding of an electrical conductor is disclosed. The core parts have elongated centre pieces with a lateral contact surface at their ends. The winding is wound directly on the core-parts without a bobbin or the-like. For manufacturing, the core-parts are arranged co-axially. After the windings have been applied, the core-parts can be arranged in a stack-like arrangement in order to form an inductive element.
In the recent years some efforts were done to integrate all three magnetic components into a single component. Some integrated magnetic structures are shown in “Integrated Magnetic for LLC Resonant Converter”, B. Yang, R. Chen, and F. C. Lee, APEC 2001, in “Design of Planar Integrated Passive Module for Zero-Voltage”, R. Chen, J. T. Strydom, and J. D. van Wyk, IAS 2001, and in US20080224809. An additional inductor winding is introduced to enhance the leakage inductance of the transformer, which replaces the series resonant inductance.
While the described patents address some proposed ameliorations, there are still some setbacks. Mostly E cores from retail or sometimes complicated core structures as in U.S. Pat. No. 6,980,077 are used. These cores are not flexible in term of mounting and of adjusting the magnetizing inductance and filtering inductance through air gap. Generally a single air gap is manufactured on the centre leg by machine and bobbins are unavoidable to wind the coils. The single air gap, the bobbin and the inflexible assembly negatively affect the costs, the power density, the power efficiency and the thermal distribution. There are supplementary power losses due to air gap fringing fields and higher winding mean length. The bobbins and single air gap are costly and cause more leakage and inductance losses. Additionally the bobbins reduce the power density and increase the thermal resistance.